As the minimum feature size in semiconductor integrated circuits shrinks, the distance between the source and drain regions becomes smaller. The reduced spacing between the source and drain regions for the field-effect transistors results in short channel effects.
To relieve the short channel effects, the semiconductor industry is constantly optimizing the fabrication processes for metal oxide semiconductor field effect transistor (MOSFET) devices. Current trends in very large scale integration (VLSI) fabrication of complimentary metal oxide semiconductor (CMOS) devices seem to focus on reducing the junction depth of the source/drain regions because shallow junctions reduce the encroachment of the source/drain depletion regions into the channel.
As CMOS technology becomes smaller, e.g., less than 50 nanometers (nm) in gate length, it becomes more and more difficult to improve the short channel device performance and at the same time maintain acceptable values for off-state leakage current.
One technique for trying to achieve this is a halo implant having extra dopant implant regions positioned next to the source and drain extension regions. The halo implant, also called a “pocket implant,” can limit the lateral diffusion of the source and drain impurities. The halo implant implants impurities having a conductivity type opposite to that of the source and drain. Usually, the halo implant comes after defining the gate and before the source/drain diffusion. However, the halo impurities also diffuse into the source/drain or the channel region during an annealing for the source/drain diffusion. This diffusion of the halo impurities may cause a threshold voltage of the transistor to fall outside of a pre-determined range, and the leakage currents may also be increased.